Conventionally, high-speed communication with large capacity has been achieved by attaching a micro-optical isolator to a semiconductor laser. As a material for the micro-optical isolator corresponding to optical communication wavebands (1.3 μm, 1.55 μm), a Bi-substituted garnet is known (for example, see Patent Document 1). In Patent Document 1, composite film is produced by containing nanoparticles of Au, Al, Ag or the like within Bi-substituted garnet thin film, thus electric polarization induced by the metal nanoparticles is increased by surface plasmon resonance of the metal nanoparticles, and thereby magnetooptic effects of the Bi-substituted garnet are increased.
Meanwhile, in relation to the Eu (II) compound (Europium chalcogenide) represented by EuO, since Eu (II) having 7 unpaired f electrons shows intense light absorption and light emission resulting from f-d transition as well as ferromagnetic property, the compound has received attention for its magnetooptical property and is expected to be available as an optical isolator material. Among the properties, particularly the Faraday effect that a polarization plane is rotated by applying a magnetic field has been the focus of attention (for example, see Patent Documents 2 and 3). Patent Documents 2 and 3 suggest that nanocrystals of EuO express magnetooptical properties by quantum size effects at room temperature.